Modern aircraft construction methods increasingly employ composite materials as structural components. Composite materials have displaced the use of aluminum in such structural components as spars, ribs, stringers, bulkheads, and aircraft skin. Additional uses of composite materials are contemplated, such as internal fuel tanks made entirely from composites.
Composite structures have numerous advantages over materials such as aluminum. Composite structures can be custom tailored to resist stresses in particular directions, whereas aluminum and other metal structural materials are generally amorphous and thus resist stress in all directions. As a consequence, a composite structure is substantially lighter than an equivalent aluminum structure. As is well known, lightweight structures are highly desirable for aircraft, as reduced weight results in increased performance. Furthermore, it is known that composite structures have a substantially longer fatigue life than materials such as aluminum. An aluminum aircraft typically has a fatigue lifetime of approximately 30,000 hours, whereas composite structures on aircraft may have fatigue lifetimes in excess of approximately 100,000 hours.
In view of the above, it would appear desirable to construct an aircraft entirely from composite materials. However, for a variety of reasons, metal components are still required in the aircraft. For example, metal fittings, such as fuel filler caps, fuel line fittings, and various fittings which penetrate composite structures, are readily available as metal components. Furthermore, these components are certified for flight. Therefore, it is desirable to use these metal components with composite structures of an aircraft.
A particular problem involving the use of metal components in a composite-structure aircraft is dissipation of electrical discharge from a natural lightning strike. In an all-metal aircraft, electrical discharge is readily distributed throughout the aircraft without arcing, because the various components generally have substantially similar electrical conductivity characteristics, coefficients of thermal expansion, etc.
Such discharges are only dangerous when electrical arcing occurs between two parts. For example, in an all-metal aircraft having an aluminum fuel tank and an aluminum fuel temperature sensor therein, any electrical arcing between the fuel tank and the fuel tank temperature sensor could result in a disastrous explosion. Electrical arcing occurs when the mechanical connection between the fuel tank and fuel sensor has a higher electrical resistance than an air gap therebetween. As in any electrical circuit, resistance heating according to Ohm's law occurs. Where an air gap between components becomes the conductive path for an electrical discharge, the resistance heating which occurs in the air gap is at a temperature which is sufficient to ignite fuel vapor in the fuel tank.
The above problem is not particularly acute in all-metal aircraft, where relatively good mechanical connection between components is easily achieved. However, electrical arcing has been known to occur where composite structures are joined to metal fittings in the presence of a current flow of up to approximately 30 kA or more. It is highly desirable that aircraft withstand current flows of up to 30 kA or more without arcing to internal components in order to be considered safe from the deleterious effects of lightning discharges.
A typical composite structure/metal component interface exists between an aluminum fuel temperature sensor 14 and a composite structure fuel tank 16, as shown in FIG. 2. The fuel tank may be constructed from a plurality of layers of graphite fabric impregnated with an epoxy resin. A bore 18 is formed in the tank to receive the fuel temperature sensor. The temperature sensor has an electrical sensor connection and bolt head 20 which resides on the outside of the fuel tank and a sensor probe 22 which extends through the bore and into a cavity defined by the fuel tank. A threaded portion 28 also extends into the fuel tank. A nut 30 is engaged with the threaded portion and secures the sensor to the fuel tank. O-rings or other sealing devices are also employed to provide a leak-proof connection.
It has been found that the above assembly repeatedly fails a 30 kA anticipated lightning threat test. In the test, electrical arcing often occurred between the composite-structure fuel tank 16 and the fuel temperature sensor 14. Gaps 26 exist in the mechanical connection between the tank and sensor, causing electrical arcing and resistance heating which could detonate fuel vapor.
In view of the above, a need exists for a method of providing a highly electrically conductive junction between metal fittings and composite structures. The resulting interface between the composite structure and metal fitting should be capable of withstanding a current flow of up to 30 kA or more without arcing.